It has previously been proposed to provide an ignition coil whose terminal ends are connected to two parallel connected spark plugs through reversely polarized diodes so that, upon selective energization of the primary of the ignition coil in different directions of current flow, selected spark plug pairs fire simultaneously, the identity of the spark plug pair firing being determined by the polarization of the respective diodes. It has been found that damage to the diodes, for example alloying-through or effective short-circuit or low-resistance paths through one of the diodes may cause firing of the spark plugs at undesired instants, for example during the intake or compression stroke of the engine, resulting in misfires and possible damage to the internal combustion engine (ICE) in which the ignition system is installed.